1. Field of the Invention
The present invention relates to an electrode for a lithium secondary battery and a method for producing the same.
2. Related Art
In recent years, development of lithium secondary batteries has been actively made. The battery properties of a lithium secondary battery, such as the charge-discharge voltages, the charge-discharge cycle life characteristics and the storage property, are greatly influenced by the electrode active material used.
Among electrode active materials capable of lithium storage and release, silicon which is a material storing lithium by being alloyed with lithium, has been examined in various aspects because it has a large theoretical capacity. However, since silicon stores lithium by forming an alloy with lithium, the volume greatly expands and shrinks with charge and discharge. This causes problems such as pulverization of the active material and separation of the material from a current collector, and thus deteriorates the charge-discharge cycle characteristics. For this reason, use of silicon has not yet been commercialized.
Electrodes for lithium secondary batteries using silicon and the like as an electrode active material and yet exhibiting a good charge-discharge cycle characteristics have been proposed (International Publication No. WO 0/31720A1), in which a microcrystalline or amorphous thin film is formed on a current collector by a thin film formation method such as a CVD method, a sputtering method and a vapor evaporation method. In such electrodes for lithium secondary batteries, the adhesion between the thin film and the current collector is good because a component of the current collector diffuses into the active material thin film appropriately, and this improves the charge-discharge cycle characteristics. For example, when a thin film made of silicon or germanium is formed on a current collector containing copper, the copper diffuses into the silicon or germanium, improving the adhesion between the thin film and the current collector.
However, since the diffusion coefficient of copper in silicon or germanium is significantly large, an excessive amount of the current collector component may diffuse in the thin film forming an alloy depending on the thin film formation conditions and the type of the current collector used. In such a case, the charge-discharge cycle characteristics may be deteriorated.